This invention relates to a surface-acoustic-wave device with an improved attenuation characteristic on the high-frequency side of the center frequency.
Devices which exploit the phenonmenon of acoustic wave propagation along the surface of an elastic solid are widely used as filters, delay lines, resonators, oscillators, amplifiers, and convolvers in electronics and communications equipment. The basic component element of a surface-acoustic-wave device is an interdigital transducer comprising a pair of interdigital comb electrodes disposed on a piezoelectric substrate. When an electrical voltage is applied across the interdigital comb electrodes, they excite acoustic waves which propagate along the surface of the piezoelectric substrate in directions perpendicular to the interdigital comb electrodes. Alternatively, acoustic surface waves propagating in these directions can be received by the interdigital transducer, which converts them to an electrical voltage signal.
A simple surface-acoustic-wave device has two such interdigital transducers: an input interdigital transducer for converting an electrical input signal to a surface acoustic wave; and an output interdigital transducer for receiving the surface acoustic wave and converting it to an electrical signal. Such a surface-acoustic-wave device acts as a bandpass filter by attenuating frequency components above and below a center frequency.
A substantial insertion loss occurs in this simple surface-acoustic-wave filter design because the input interdigital transducer generates surface acoustic waves in two opposite directions, but only the acoustic surface wave propagated toward the output interdigital transducer is received. The insertion loss can be reduced by arranging a plurality of input interdigital transducers and a plurality of output interdigital transducers in alternating sequence in a row on the piezoelectric substrate, with input interdigital transducers at both ends of the row. In this configuration, the general formula for the inertion loss is 10 log(N/M) decibels, where N is the number of input interdigital transducers and M is the number of output interdigital transducers (M=N-1). A prior-art device with three input interdigital transducers and two output interdigital transducers, for example, has an insertion loss of only 10 log(3/2) or substantially 1.76 dB.
A problem in prior-art devices of this design is that while increasing the number of interdigital transducers reduces the insertion loss, it also reduces the amount of attenuation in the attenuation bands.